getArgToFuncMapping

Allow a UDF to specify type specific implementations of itself. For example,
an implementation of arithmetic sum might have int and float implementations,
since integer arithmetic performs much better than floating point arithmetic. Pig's
typechecker will call this method and using the returned list plus the schema
of the function's input data, decide which implementation of the UDF to use.

A List containing FuncSpec objects representing the EvalFunc class
which can handle the inputs corresponding to the schema in the objects. Each
FuncSpec should be constructed with a schema that describes the input for that
implementation. For example, the sum function above would return two elements in its
list:

allowCompileTimeCalculation

Whether the UDF should be evaluated at compile time if all inputs are constant.
This is applicable for most UDF, however, if a UDF will access hdfs file which
is not available at compile time, it has to be false